CF TIP: Determing if a UDF argument is "null"

Ok, so the title is misleading. CF doesn't really have a concept of
null, but it does have the concept of "not exists."

If you need to differentiate between when an argument of a function
has been supplied or not, set the required attribute
to true and leave the default attribute
off. If the argument is not supplied to the function, then a against
structKeyExists(arguments, "argumentName") will return false.
If a value has been supplied, the function will return true.

There are times when using the default attribute to
just supply a value is fine, but there are many times when you
need to differentiate between when an argument was actually supplied
or when it was not.